A higher speed and higher reliability have been stringently demanded of a copying device or a printer in recent years. Accordingly, such machine has started to be constituted of elements additionally simple in various respects. As a result, performance demanded of a developer has become additionally sophisticated; an excellent machine will not be established unless an improvement in performance of a developer is achieved.
Of the methods each involving developing an electrostatic charge image with toner, a two-component development method involving the use of a two-component developer obtained by mixing toner with a magnetic carrier has been suitably employed in a full-color copying machine or printer where high image quality is requested. In the two-component development method, the magnetic carrier imparts an appropriate, positive or negative charge amount to the toner by triboelectric charging, and the surface of the magnetic carrier is caused to carry the toner by the electrostatic attraction of the triboelectric charging.
Various properties are demanded of the magnetic carrier and the toner of which the above two-component developer is constituted. Examples of properties particularly important for the magnetic carrier include appropriate chargeability, resistance to an alternating voltage, impact resistance, wear resistance, spent resistance, and developability.
In association with the spread of, for example, a high definition or full-color printer, an additional improvement in image quality is requested even in electrophotography nowadays. However, in the case where an alternating electric field is applied for an improvement in image quality, when the resistivity of a magnetic carrier is low, a latent image electric potential leaks through the magnetic carrier, thereby making it impossible to obtain a good image. Accordingly, the magnetic carrier must have some degree or more of resistivity. Therefore, when the magnetic carrier is conductive, the magnetic carrier is preferably coated with a magnetic carrier coating agent so as to be used as a coated magnetic carrier. A possible approach to bringing the resistivity of a magnetic carrier to an appropriate value involves coating the surface of a core material with an insulative resin. The approach can achieve an improvement in strength of a core and improvements in durability and charging stability simultaneously with the achievement of an appropriate value for the resistivity.
In addition, a capacity for a developer has been reduced in association with a reduction in size of a developing unit, and the speed at which the developer is stirred has increased owing to an increase in speed at which an image is outputted. As a result, a burden on the developer in the developing unit tends to be additionally heavy. The burden on the developer is the impact of magnetic carriers or of a magnetic carrier and toner upon stirring or the impact of a magnetic carrier and a developer layer thickness regulating member for forming a developer layer having a predetermined thickness on a developing sleeve. The impartment of appropriate charge to the toner cannot avoid any one of those impacts, and various proposals have been made to alleviate those impacts. For example, the flowability of a developer is improved by controlling the shape of a magnetic carrier, whereby coat peeling is alleviated (see JP 08-292607 A). However, the control of only the shape of the magnetic carrier in this way does not provide the developer with a completely satisfactory lifetime. Further, a recess of a magnetic carrier core is filled with a conductive additive resin for a reduction in specific gravity and the control of a resistance, whereby an increase in lifetime of a developer and the prevention of the adhesion of a magnetic carrier are achieved (see JP 2002-278165 A). However, when a lower layer component of a magnetic carrier coating layer is conductive as described above, an image failure due to fog occurs owing to the deterioration of durability.
Meanwhile, in order to solve the above problems, a so-called automatic magnetic carrier refresh (ACR) developing device has been disclosed (see, for example, JP 02-21591 B). In the device, simultaneously with the replenishment of a developing unit with toner to be consumed by development, the developing unit is replenished with a new magnetic carrier, and the remaining old magnetic carrier in the developing unit is discharged so that a deteriorated magnetic carrier is gradually replaced. As a result, an increase in lifetime of a developer is achieved.
Further, the specific gravity of a magnetic carrier is reduced so that an external additive is prevented from being embedded in the toner, and an increase in lifetime of a developer is achieved by an ACR development mode (see, for example, JP 2001-330985 A).
In addition, a magnetic carrier core agent having irregularities is filled with a coating resin, whereby a reduction in specific gravity of a magnetic carrier and the smoothness of a fine surface are achieved, and, furthermore, an increase in lifetime of a developer and high image quality are achieved (see, for example, JP 2003-156887 A).
As described above, various proposals have been made to achieve an increase in lifetime of a developer and high image quality. However, the incorporation of a magnetic carrier into a developer for replenishment deteriorates the flowability of the developer, so the replenishment amount of the developer for replenishment to flow from a container for storing the developer for replenishment into a developer tank is not always constant, that is, the amount is uneven. Accordingly, each of the proposals is still susceptible to improvement because each of them provides an image with poor density stability.
A developer for replenishment has been requested, which: can cope with recent, stringent demands for quality on various objects such as a fine line, a small letter, a photograph, and a color copy; and satisfies high image quality, high stability, a high speed, and high durability while coping with a reduction in size of a main body and the simplification of the control of the main body.